A power supply of an electronic device such as a television, a smart phone, a PC and the like should stably supply a constant direct-current voltage. As the power supply, a switching power-supply device is used (for example, refer to JP-A-2013-63003).
A switching power-supply device uses, as a switching element, a semiconductor device such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor), a thyristor and the like and converts an input direct-current voltage into an alternating-current voltage by an on-off operation of the switching element. The converted alternating-current voltage passes through a transformer, a rectification circuit and a smoothing circuit in that order, is converted into a stable direct-current voltage and is then output.
In the switching power-supply device, a switching frequency is controlled depending on the output voltage, thereby stabilizing the output voltage. However, in a situation where the output voltage is largely changed (for example, lowering of the input voltage due to instantaneous interruption or power failure or heavy load), even when the switching frequency is reduced, it is difficult to constantly maintain the output voltage.
According to the switching power-supply device disclosed in JP-A-2013-63003, a front stage of a DC-DC converter is provided with a boost circuit, and a setting voltage of the boost circuit is set to be a rated input voltage or lower. Thereby, the boost circuit stops a switching operation while the rated input voltage is input, thereby reducing a switching loss in the boost circuit. Also, when the input voltage is lowered due to the power failure and the like, the boost circuit starts the switching operation to thus constantly maintain the voltage to be input to the DC-DC converter of a rear stage. Thereby, an input voltage range of the DC-DC converter connected to the rear stage is narrowed to enable a design of high efficiency.